Biosynthesis and Degradation of Thyroid Hormones

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Description

Iodide, which is ingested in food and water, is actively concentrated by the
thyroid gland, converted to organic iodine by TPO (Thyroid Peroxidase also known
as Iodide Peroxidase), and incorporated into tyrosine in intrafollicular
thyroglobulin within the colloid at the basal cell surface of the thyroid
follicular cell. The tyrosines are iodinated at one (Monoiodotyrosine) or two
(Diiodotyrosine) sites and then coupled to form two active hormones, Thyroxine
(T4, a Tetra-Iodinated Tyrosine derivative) and Triiodothyronine (T3). Unlike
the other endocrine hormones, these are not peptides. They are derived from the
amino acid tyrosine. T4 is the major active hormone. In tissues outside the
thyroid, particularly in the liver and kidney, T4 is converted to T3, an active
metabolite, by monodeiodination [...]